Python dictionary is a collection of key-value pairs, where each key is unique and associated with a value. Dictionaries are mutable, meaning they can be changed after they are created.

Syntax

A dictionary in Python is defined using curly braces {} and contains key-value pairs separated by a colon :. Each key-value pair is separated by commas.

dictionary_name = {key1: value1, key2: value2, ...}

Create a Dictionary

To create a dictionary, you can use the curly braces {} and separate the key-value pairs with colons :.

person = {"name": "John", "age": 30, "city": "New York"}
print(person)  # Output: {'name': 'John', 'age': 30, 'city': 'New York'}

or

# Creating an empty dictionary
empty_dict = {}

# Adding key-value pairs to the dictionary
empty_dict["name"] = "Alice"
empty_dict["age"] = 30
empty_dict["city"] = "New York"

# Accessing and printing the dictionary
print(empty_dict)

You also can use dict() method to create a dictionary.

Example:

# Creating a dictionary using dict() constructor with key-value pairs
dict_constr_1 = dict(name="Bob", age=25, city="San Francisco")

# Creating a dictionary using dict() constructor with a list of tuples
dict_constr_2 = dict([("key1", "value1"), ("key2", "value2"), ("key3", "value3")])

# Accessing and printing the dictionaries
print(dict_constr_1)
print(dict_constr_2)

Output:

{'name': 'Bob', 'age': 25, 'city': 'San Francisco'}
{'key1': 'value1', 'key2': 'value2', 'key3': 'value3'}

Accessing and Updating Values

You can access values in a dictionary using keys and update them as needed.

# Accessing and updating values
print(my_dict["name"])  # Output: Alice
my_dict["age"] = 31

Add a Key-Value Pair

To add a new key-value pair to a dictionary, you can use the assignment operator = to assign a value to a new key.

person = {"name": "John", "age": 30, "city": "New York"}
person["country"] = "USA"
print(person)  # Output: {'name': 'John', 'age': 30, 'city': 'New York', 'country': 'USA'}

Remove a Key-Value Pair

Here are two methods to remove a key-value pair in a dictionary.

del

person = {"name": "John", "age": 30, "city": "New York"}
del person["city"]
print(person)  # Output: {'name': 'John', 'age': 30}

pop()

person = {"name": "John", "age": 30, "city": "New York"}
city = person.pop("city")
print(person)  # Output: {'name': 'John', 'age': 30}
print(city)  # Output: New York

Iterate over a Dictionary

To iterate over a dictionary, you can use a for loop.

Example:

person = {"name": "John", "age": 30, "city": "New York"}
for key in person:
    print(key, person[key])

Output:

name John
age 30
city New York

Alternatively, you can use the items() method to iterate over both the keys and values.

Example:

person = {"name": "John", "age": 30, "city": "New York"}
for key, value in person.items():
    print(key, value)

Output:

name John
age 30
city New York

Dictionary Operations

Here are some common dictionary operations:

Get a value

Use the get() method to retrieve a value by its key.

person = {"name": "John", "age": 30, "city": "New York"}
print(person.get("name")) # Output: John

Check if a key exists

Use the in operator to check if a key exists in the dictionary.

person = {"name": "John", "age": 30, "city": "New York"}
print("name" in person) # Output: True
print("email" in person) # Output: False

Get all keys

Use the keys() method to get all the keys in the dictionary.

person = {"name": "John", "age": 30, "city": "New York"}
print(person.keys()) # Output: dict_keys(['name', 'age', 'city'])

Get all values

Use the values() method to get all the values in the dictionary.

person = {"name": "John", "age": 30, "city": "New York"}
print(person.values()) # Output: dict_values(['John', 30, 'New York'])

Get all items

Use the items() method to get all the key-value pairs in the dictionary.

person = {"name": "John", "age": 30, "city": "New York"}
print(person.items()) # Output: dict_items([('name', 'John'), ('age', 30), ('city', 'New York')])

Clear a dictionary

Use the clear() method to remove all key-value pairs from the dictionary.

person = {"name": "John", "age": 30, "city": "New York"}
person.clear()
print(person) # Output: {}

Copy a dictionary

Use the copy() method to create a copy of the dictionary.

person = {"name": "John", "age": 30, "city": "New York"}
person_copy = person.copy()
print(person_copy) # Output: {'name': 'John', 'age': 30, 'city': 'New York'}

Merge two dictionaries:

Use the update() method to merge two dictionaries.

person1 = {"name": "John", "age": 30}
person2 = {"city": "New York", "country": "USA"}
person1.update(person2)
print(person1) # Output: {'name': 'John', 'age': 30, 'city': 'New York', 'country': 'USA'}